Steve,Thanks for the advice guys.
I'm pretty sure that my roof lights are 400 x 400 MPK.
The only place I can imagine the screws are is behind plastic caps on the bottom of the legs of the pop up screen. I've tried to prise them off but they will not budge. Are these the ones you described John or are there other screws I cannot see? Just want to make sure I'm barking up the right tree before I go and break the thing in vain!
Thanks again